Tough Smashup (Blue)

When this defends, clash with the attacking hero. The winner creates a Toughness token. You may put your revealed card on the bottom of its owner's deck.

Rulings

From the Super Slam release notes

Tough Smashup’s ability triggers when it becomes a defending card on the combat chain. You will clash with the attacking hero when this triggered effect resolves. The winner creates a Toughness token after the outcome of the clash is determined. Regardless of if you win or lose, after the winner creates a toughness token, you may choose to put your revealed card on the bottom of its owner’s deck. The attacking hero doesn’t put their revealed card on the bottom of the deck from this effect.
If you were to reveal the top of the opponent’s deck for the clash (e.g. The Old Switcheroo), you may choose to put that card on the bottom of its owner’s deck.
When resolving Tough Smashup’s triggered effect, there is no priority between any of the effects generated (e.g. clashing with the attacking hero, creating a Toughness token, putting a card on the bottom of the deck.).
When resolving Tough Smashup’s triggered effect, if there is no attacking hero when this triggered effect resolves, the clash effect fails and there is no winner. Because you didn’t reveal a card from this effect, you would not put any cards on the bottom of the deck.

Keyword notes

Clash - Clashing heroes reveal the top card of their deck. The hero that reveals the card with greatest {p} wins.

Clash is an effect keyword.
Effects that modify values of cards in your deck are considered when clashing (e.g. Kayo, Mutated Mass, etc.)
If you win a clash, the opponent is considered to have failed to win the clash.
If no one wins a clash, you’re both considered to have failed to win the clash.
If both you and the opponent reveal a card with the same , no one wins the clash.
If both you and the opponent reveal a card without and/or don’t reveal any card, no one wins the clash.
If you reveal a card with and the opponent reveals a card without , you win the clash.
If you reveal a card with and the opponent doesn’t reveal any card, you win the clash.